1. Field of the Invention
The present invention relates to a device for creating a reference humidity that includes a saturation chamber, a downstream arranged condensate cooler and a measuring chamber connected with the condensate cooler, in which a desired reference humidity can be definitely set by varying pressures and/or temperatures.
2. Discussion of Related Art
Two basically different methods for calibrating humidity measuring apparatus (for example dewpoint hygrometers for measuring the dewpoint temperature or hygrometers for measuring the relative humidity) are known at the present time and are discussed below.
Comparison Method
Here, a device for creating reference humidity, or a system in general, is utilized, by which gas or air of a predetermined, as stable as possible relative humidity or dewpoint temperature is created (“humidity representation”). A reference measuring device is required in addition, which measures the actual humidity, and whose reading is compared with the test specimen.
Relatively simple systems for representing the humidity are generally considered for the comparison method, which only need to assure sufficient stability.
Reference Method
A system, or a device, which generates gas or air of a defined temperature or a defined reference humidity, is also needed with this system. However, in contrast to the comparison method, no additional reference measuring device is required, instead the system provides the humidity or the dewpoint temperature of a known value with such high accuracy (or low uncertainty) and stability that no reference measuring device is necessary. With a correct layout of the system, the stability and accuracy of the humidity representation is clearly better than that of the known reference measuring devices.
In connection with the reference method, so-called 2-pressure/2-temperature humidity generators are customarily employed as the devices for creating reference humidity, such as they are known from EP 0 989 373 A2 or from the publication entitled “The NBS Two Pressure Humidity Generator, Mark 2”, by S. Hasegawa and J. W. Little, J. Res. Nat. Bur. Stand. 81A, No. 1, pp. 81 to 88 (1977). Their functional principle will be briefly explained in what follows.
Dry air or nitrogen at a pressure p1 is conducted here via a pressure regulator through a water bath in a saturation chamber (pre-saturating device) with the temperature ts. As a result, practically water vapor-saturated air at an approximate dewpoint temperature ˜ts at a pressure p1 is obtained.
Subsequently, the saturated air is conducted through a heated connecting line with a temperature tv1>ts into a condensate cooler. The condensate cooler is temperature-stabilized in a highly stable liquid bath and cools the inflowing air to the condenser temperature t1, wherein the entire amount of excess water appears as the condensate. As a result, air at a pressure p1 and a temperature t1 is obtained, which is completely saturated with water vapor. The pressure p1 and temperature t1 are measured directly in the condensate cooler.
The partial water vapor pressure ew′(t1, p1) of the air in the condensate cooler is obtained by means of the following equation (1)ew′(t1, p1)=ew(t1)*f(p1, t1)  Equ. (1)wherein
ew(t): saturation vapor pressure of water at the temperature t in the pure phase in accordance with the following reference: Sonntag, D., “Important New Values of the Physical Constants of the 1986 Vapour Pressure Formulations based on ITS-90 and Psychometer Formulae”, Z. Meteorol70 (1990) 5, pp. 340 to 344 (“the Sonntag reference”).
f(p,t): actual gas correction (“enhancement factor”) of air in accordance with the following references: 1) B. Hardy, “ITS-Formulations for Vapour Pressure, Frostpoint Temperature, Dewpoint Temperature and Enhancement Factors in the Range −100 to 100° C.”, Third International Symposium on Humidity and Moisture (1998), pp. 214 to 221 (“the Hardy reference”) and 2) Greenspan, L., “Functional Equations for Enhancement Factors for CO2-Free Moist Air”, J. Research NBS, A. Physics and Chemistry 80A (1976), pp. 41 to 44 (“the Greenspan reference”).
ew′(p,t): saturated vapor pressure of the actual system in the presence of air or N2.
The saturated air is removed from the condensate cooler through a heated connecting line with a temperature tv2>t1, is relaxed to the pressure p2 via a heated needle valve with the temperature tp>tv2 and is conducted through a heated connecting line with the temperature tv3>tv2 to a consumer or measuring chamber, wherein the gas pressure p2 is measured in the measuring chamber.
The partial water vapor pressure e at the location of the consumer then results ase=(p2/p1)*ew′(p1, t1)  Equ. (2)and the dewpoint temperature td from the equation (3):e=ew′(p2, td)  Equ. (3)
The system is operated at a continuous gas flow wherein, with the right construction, the dewpoint td of the gas flow results merely from the measurement of the two pressures p1 and p2 and the condensate cooler temperature t1.
By varying the condensate cooler pressure p1 and the condensate cooler temperature t1 it is possible to obtain minimal dewpoint temperatures of −27° C. at a maximum pressure of 9400 mbar and a temperature t1=0.2° C.
Vice versa, it is possible to obtain a maximum dewpoint temperature of 90° C. at a temperature t1=100° C. and a pressure p1=1430 mbar.
By subsequently introducing the gas flow into a measuring chamber with the temperature t2 it is possible to obtain a relative humidity UwUw=(e/e′w(p2, t2))*100  Equ. (4)in it.
In this way it is possible to achieve minimal dewpoint temperatures up to approximately −27° C. for positive temperatures t1. For lower dewpoint temperatures it would be necessary to considerably increase the condensate cooler pressure p1 (p1=100 bar for td˜−50° C., p1=400 bar for td˜−60° C.), however, this results in technical problems regarding sturdiness and safety. But it is decisive that, based on the available experimental measurement data known in the literature, the applicability range of the functions of the Sonntag, Hardy and Greenspan references mentioned previously is limited to pressure ranges below p1=20 bar.
Therefore the humidity representation for low dewpoint temperatures is no longer performed by the saturation vapor pressure with respect to water, but to ice, which leads to similar functions. The humidity representation with respect to ice is customarily obtained by two known methods.
In a first variation, dry gas is saturated over an ice layer, wherein the ice layer can be embodied as a covering of ice on helical tubes, for example, which are stabilized to a temperature of t1<0° C. In connection with this, a difficulty typically arises of producing an appropriate ice layer in the helical tubes. Moreover, the life of such a system with the amounts of ice at the tube walls, problems can arise in general because of flow losses in the helical tubes in the form of pressure losses, which in turn can lead to significant measuring errors. At the end of the life of the ice layer it is necessary to thaw the system, and a fresh covering of the tube walls with ice must be provided.
In a second variation, excess moisture is removed from moist air by sublimation in a condenser. Essentially the same process is performed as in a condensate cooler wherein, however, in contrast thereto ice is precipitated. But this method has two substantial disadvantages.
The ice is mainly precipitated at the inlet side, which very quickly leads to the condensate cooler freezing up. Significant pressure losses occur very rapidly in the condensate cooler, which in turn lead to corresponding measurement errors. Furthermore, the ice customarily does not precipitate in the form of a massive smooth layer of ice, but in the form of hoarfrost, which has only slight sturdiness and mechanical stability. The ice needles being created in the course of this have a tendency to be carried along in the gas flow, which in turn leads to an erroneous representation of the humidity.
Regarding humidity representation methods by the 2-pressure/2-temperature systems, there are different variations, which substantially always aim toward a simplification of the basic principle. For example, the two temperatures t1 and t2 can be identical, and a purely 2-pressure system is obtained, wherein the relative humidity is only adjusted by the pressure. If, however, both pressures p1 and p2 are selected to be identical, a 2-temperature system results for the representation of the relative humidity. In the case of a dewpoint representation, such a system then is also called a 1-pressure generator, in which the dewpoint representation takes place only by the condenser temperature.
The systems can operate with a continuous gas flow, as described above. However, there are also systems (2-temperature generators, or 1-pressure generators), which are operated with a circulating gas flow.
Independently of the respective embodiment, all variations are however based on the same basic principle. With all systems, the construction of the condensate cooler or condenser between the saturation device and the consumer has a central importance. Although systems exist for all dewpoint ranges, which operate with great accuracy, completely different systems are customarily required for different dewpoint ranges which, moreover, can have substantial disadvantages in their operation (pressure loss, short service life, limitations at high temperatures).
Several known embodiments of the condensate cooler now exist.
Plate condensers or fin-type condensers are known, wherein the gas flow is conducted up and down in a meander shape in a plate condenser, in fact vertically. In this connection reference is made for example to the publication by A. Scholz “A Standard Calibrator for Air Hygrometers”, OIML Bulletin No. 97, December 1984, pp. 18 to 27. In such condensate coolers, excess water is precipitated at the bottom of the condenser and must be removed relatively often, since too high a water level leads directly to pressure losses and therefore to measurement errors.
Condensate coolers in the form of helical tube coolers are furthermore known. Here, the gas flow is conducted in a helical tube through cooling medium, wherein the excess water is precipitated at the tube walls and flows into a water precipitator. This also must be periodically emptied.
Normally, these condenser structures can only be used for an actual condensate operation, they are only useful in a limited way for a humidity representation over ice and for a limited time, since the ice is precipitated immediately at the input side and plugs up the condenser.